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More misconceptions
Friction and electrical resistance
Catherine LowRangi Ruru Girls’ School
Four forces act on Toni and the bike while she rides: weight, support, friction and push.
On the diagram below, draw in arrows to show the directions of ALL FOUR forces acting on Toni and the bike. Label the forces.
FRICTION PUSH
SUPPORT
WEIGHT /GRAVITY
FRICTIONPUSH
SUPPORT
WEIGHT /GRAVITY
FRICTION PUSH
SUPPORT
WEIGHT /GRAVITY
FRICTIONPUSH
SUPPORT
WEIGHT /GRAVITY
FRICTION
Static
• Surface friction
Kinetic
• Fluid friction/Drag
Common phrase:
FRICTION OPPOSES MOTION
More correct:
(surface) FRICTION OPPOSES SLIDING
Drag opposes the relative motion of the object through the air
KINETIC FRICTION
STATIC FRICTION
sliding
Kinetic friction
Direction object WOULD slide if there was no grip
Static friction
DRAGSTATIC FRICTION
SUPPORT
WEIGHT /GRAVITY
FRICTION PUSH
Accelerating
braking
Constant speed
Rolling resistanceDirection of motionSupport force
Rolling resistanceDirection of motion
Overall support force
SUPPORT
WEIGHT /GRAVITY
DRAGSTATIC FRICTION
• DRAG
• RESISTANCE
• STATIC FRICTION• GRIP•TRACTION
SUPPORT
WEIGHT /GRAVITY
DRAGSTATIC FRICTION
SUPPORT
WEIGHT /GRAVITY
Explain how the effect of the spikes on the sport shoes changes the effect of forces acting on the athlete and how this leads to an improvement in performance.
Answer: Spikes on shoes increase the effect of friction. Resulting in a greater reaction/push/thrust force.
ELECTRICAL RESISTANCE
R = Slope of the V vs I graph
OR
R = dVdI
R = VI
Ohmic conductors – not an issue
V
I
The diagram below is a graph showing the voltage – current characteristics of a particular diode.
State what physical quantity is represented by the gradient of the graph line.
Answer: Resistance
Resistance = how hard it is to push charges through the device
R = VI
V
I
V
I
V
I
P = I2R
P =V
R2
Halliday, Resnick, Walker
Serway
Direction you want to go
Direction of sliding
STATIC FRICTION